There are many known bag machines. One style is a rotary drum machine. Rotary drum machines are well known, and found in U.S. Pat. Nos. 6,117,058, 4,934,993, 5,518,559, 5,587,032 and 4,642,084 (each of which is hereby incorporated by reference). Bag machine, as used herein, includes a machine used to make bags such as draw tape bags, non-draw tape bags, and other bags.
A detailed description of the operation of rotary bag machines may be found in the patents above, or in prior art commercially available machines such as the CMD 1270GDS or 1552ED, but their general operation may be seen with respect to FIG. 1. A prior art rotary bag machine 100 continuously processes a web 201 using a dancer assembly 203, a pair of drum-in rolls 205 and 206 (203-206 are part of an input section). A sealing station includes a sealing drum 208, a pair of drum-out rolls 210 and 211, and a sealing blanket 213. A perforating station includes a pair of knife-in rolls 215 and 216, a knife 218 (which could be any other web processing device such as a perforator, knife, die cutter, punching station, or folding station, prior to 215/216), a pair of knife-out rolls 219 and 220 (210-220 are part of an output section), and a controller 221. Perforating station, as used herein, includes a device that perforates a film. Input section, as used herein, includes the portion of a bag machine where the web is received, such as an unwind and a dancer assembly. Output section, as used herein, includes processing stations that act on a web downstream of the seals being formed, such as winders, folders, etc. Processing station, as used herein, includes any device that operates on the film, such as sealing, folding, perforating, winding, etc. Sealing station, as used herein, includes a device that seals a film
The web is provided through dancer assembly 203 to drum 208. Drum 208 includes a plurality of seal bars 209. The seal bars are heated and create the seals forming the bags from web 201. The distance between seals created by the drum is related to the bag length (for bags formed end to end) or the bag width (for bags formed by making side seals). End to end bags are formed with one seal from the drum, and side to side bags are formed with a pair of seals. The drum diameter may be adjusted and/or less than all of the seal bars turned on to determine the distance between seals, and hence bag size.
Generally, rotary motion machines registers a downstream rotary knife to perforate between two seals, or beside a seal. Variations due to tension, film gauge variation, machine variations etc., occasionally causes seals to get cut off, or the distance between a seal and perforation to be too great.
The prior art of FIG. 1 provides that after web 201 leaves drum 208 it is directed to rotary knife 218, which creates a perforation between bags, or could separate adjoining bags. When the bags are end to end bags the perforation is placed close to the single seal such that when the bags are separated, the perforation and the perforated end is the top of one bag, and the seal is the bottom of the adjoining bag. Ideally, the perforation is close to the seal to reduce waste, although this is difficult in practice. The distance between the seal and the perforation is called the skirt length. When bags are formed side to side, the perforation is made between the pair of seals. Thus, there are skirt lengths on either side of the perforation. A seal is needed on both sides of the perforation, since the side of both bags should be sealed. The web between the pair of seals is wasted. Thus, the pair of seals should be close to one another to reduce waste, although this is also difficult in practice.
Controller 221 is connected to the various components to control speed, position, etc. Sensors may be used to sense print on the web to form the seals and/or register the perforation to the seal (place it in the correct location with respect to the seal). Also, sensors may detect seals prior to the formation of the perforation to try and form the perforation in the correct location. Sensing the seal has proven to be difficult. One prior art example of a system that sensed seals is described in U.S. Pat. No. 6,792,807, hereby incorporated by reference. Another prior art patent, U.S. Pat. No. 5,447,486 uses printed marks or marks created by the seal bar to sense the location of the seal to try and register the perforation to the registration. If the perforation is placed too close to one side seal, then the seal may be cut off, rendering the bag useless.
The prior art teaches open loop control. The sensors attempt to sense the location of the seal, and then attempt to control the perforator to place the perforation in the proper location. However process variations can cause the registration to be incorrect. The prior art does not teach to close the loop and determine if the perforations was actually made in the intended location.
Accordingly, a method and machine for making bags that allows for closed loop control of the seal and perforation registration is desirable.